Upgrading a power-splitting passive optical network using optical filtering

ABSTRACT

The passive optical network system and method for providing a predetermined wavelength of data to remote users according to the present invention includes a multiple wavelength transmitter for transmitting a multiwavelength signal. The multiwavelength signal is provided by an access provider and has a plurality of signal components each of predetermined wavelength. A power-splitting passive optical network receives and power-splits the multiwavelength signal into a plurality of distributed multiwavelength signals each associated with a respective remote user. A filter selectively filters out, for each remote user, ones of the signal components of the associated distributed multiwavelength signal to provide the remote user with a selected one signal component of predetermined wavelength.

FIELD OF THE INVENTION

The present invention relates in general to a passive optical network(PON) system. In particular, the present invention describes a multiplewavelength transmitter in a central office that sends multiplebandwidths to a power-splitting PON and a filter incorporated in each ofthe remote optical network units (ONUs) such that each ONU in the PONreceives only one selected bandwidth.

BACKGROUND OF THE INVENTION

The desire to have high capacity information conduits reachingresidential customer premises has promoted intense interest in broadbandtransmission over copper cable, wire, wireless, and optical fiber media.Fiber-to-the-home, in which optical fiber transport is used over theentire path, is appealing for its large information capacity. Varioustechniques are available for separating different services fortransmission over the same lines, for example the transmitted signalsmay be time, wavelength, or sub-carrier frequency multiplexed.

Passive optical networks (PONs) are architectures in which there are nointervening active components between the host digital terminal orcentral office (CO) and customer premises. PONs are desirably installedinto remote units, such as homes, to provide data such as video andaudio and the like over a fiber.

In other words, PONs require no active components for directing opticalsignals between the CO and a network subscriber's terminal equipment.Passive optical networks, therefore, require no power or processing inthe field to direct optically encoded information to its destination.Typically, a PON includes a first fiber star formed as a plurality ofoptical paths extending from the CO to a remote node. Downstream opticalsignals are transmitted from the CO to the remote node, where the signalis passively split and distributed to one of a plurality of units ofnetwork subscriber equipment. The network units may transmit opticallyencoded signals upstream to the remote node to form a multiplexed signalfor distribution to the CO. Lasers are generally used to generate lightused to form the transmitted light signals.

A standard PON model is shown in FIG. 1, and consists of a first fiberstar 1, typically a plurality of optical fibers 2 extending from acentral office 4, to one of a plurality of remote nodes 6, i.e., RN₁,RN₂, . . . RN_(N). Downstream signals are transmitted from the CO 4towards the remote node for further distribution. At the remote nodes,light is passively split and distributed via a plurality of opticalfibers 8 (a second star) to a plurality of optical network units (ONUS)10, i.e., ONU₁, ONU₂, . . . ONU_(N). The ONUs 10 provide service to oneor more end users wherein each downstream optical signal is received andelectronically distributed to end users. The ONUs 10 may transmitupstream signals which are combined at the remote node. Each remote node6 (or passive star) passively combines transmissions from the ONUs 10onto a single optical fiber 2 for distribution to the CO.

Two passive optical network architectures are a telephony over passiveoptical network (TPON) and a wavelength division multiplexing passiveoptical network (WDM PON). In a TPON architecture, a CO broadcasts adownstream optical signal to all ONUs using time division multiplexing(TDM) protocol. A laser with a common wavelength band, requiringsynchronization, may also be used. TDM typically includes a frame ofinformation subdivided into time slots assigned to individual ONUs.

Wavelength division multiplexing (WDM) is a technology in which multiplewavelengths share the same optical fiber in order to increase thecapacity and configurability of networks. WDM generally increasesoptical system capacity by simultaneously transmitting data on severaloptical carrier signals at different wavelengths. The total systemcapacity is increased by a factor equal to the number of differentwavelength channels. WDM PONs utilize an architecture within which eachONU or subscriber is assigned a unique wavelength by the central office.Signals destined for each remote node (and ultimately, each opticalnetwork unit) are created by modulating light at N distinct wavelengthsat the CO. The modulated light is multiplexed onto a fiber directed tothe remote node. The downstream signals are split and distributed to theONU as a function of wavelength within a wavelength divisiondemultiplexer at the remote node. In the upstream transmission direction(optical network unit to remote node), the light is transmitted atassigned wavelengths, typically by a laser.

Compared to TDM PONs, WDM PONs have the advantage that they do notbroadcast individual subscribers' data to all premises. As a result,privacy is enhanced and the electronics in the ONU need only operate atthe subscriber's data rate. However, upstream transmission through awavelength routing device can be difficult. Temperature-controlledsingle-frequency lasers at each home are impractical. Spectral slicingof light emitting diodes and the use of modulators combined with anoptical loopback have also been used. Another approach is to uselow-cost, uncooled Fabry-Perot lasers at the home and combine them atthe remote node with a passive splitter. This approach is costly becauseit requires extra fiber and an extra passive component (a WDM splitter)at the remote node, unless a single passive device can accomplish bothdownstream wavelength routing and upstream power combining. Thus,wavelength division multiplexing, with different services on differentwavelengths, requires additional optical transmitters and receivers tobe installed wherever an expansion of services and additional channelsis required. Each remote unit is assigned a different frequency using awavelength router. However, due to a number of technical problems, thisWDM system is not commercially viable for mass market applications likefiber distribution to the home. One such problem is the small number ofchannels currently accommodated. Present multichannel laser diodes arevery difficult to fabricate with acceptable yield even with as few aseight channels. In addition, passive WDM splitters currently availablehave a large temperature variation of their passband channels, therebyrequiring a continuous tunability in the multichannel sources that hasnot yet been achieved.

Another type of PON is a power splitting PON (PSPON) which is used witha single-wavelength TDM-encoded transmitter in the central office. Onefiber from the CO is directed into a standard power splitter instead ofa wavelength splitter. Thus, each remote unit gets a fraction of thetotal power. This is a time domain multiplexing protocol in which allremote units get the same data, but only the data intended for theparticular remote unit is retrieved by the remote unit using an ID code,for example. These wavelength-independent PSPONs utilize time divisionmultiplexed access (TDMA) for signaling in both directions and passiveoptical splitters for branching, thereby achieving low cost, whilecompromising power budget, signaling integrity, and security.

Although the art of transmitting data from a central office to a remoteunit is well developed, there remain some problems inherent in thistechnology. One particular problem is efficiently providing data atdifferent wavelengths to different remote units. Therefore, a needexists for a system that provides data at different wavelengths todifferent remote units that is less expensive and complex than thoseusing a WDM splitter.

SUMMARY OF THE INVENTION

The passive optical network system and method for providing apredetermined wavelength of data to remote users according to thepresent invention includes a multiple wavelength transmitter fortransmitting a multiwavelength signal. The multiwavelength signal isprovided by an access provider and has a plurality of signal componentseach of predetermined wavelength. A power-splitting passive opticalnetwork receives and power-splits the multiwavelength signal into aplurality of distributed multiwavelength signals each associated with arespective remote user. A filter selectively filters out, for eachremote user, ones of the signal components of the associated distributedmultiwavelength signal to provide the remote user with a selected onesignal component of predetermined wavelength.

The foregoing and other aspects of the present invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a conventional passive optical network model.

FIG. 2 is an exemplary network system in accordance with the presentinvention.

FIG. 3 is a diagram of another exemplary network system in accordancewith the present invention.

FIG. 4A shows a chart showing transmission vs. wavelength for an accessprovider in an exemplary network system.

FIG. 4B shows a chart showing transmission vs. wavelength of filtereddata received at a remote unit.

FIG. 5 shows an exemplary access card in accordance with the presentinvention.

FIG. 6 shows an exemplary optical network unit in accordance with thepresent invention.

FIG. 7 shows further detail of the exemplary optical network unit shownin FIG. 6.

FIG. 8 shows a further exemplary device in accordance with the presentinvention.

FIG. 9 is a diagram of a conventional wavelength division multiplexedpassive optical network.

DESCRIPTION OF EXEMPLARY EMBODIMENTS AND BEST MODE

The present invention is directed to upgrading an existingpower-splitting passive optical network (PSPON) to act as an effectivefull wavelength division multiplexed (WDM) PON. Instead of sending onewavelength down the fiber with a single-wavelength TDM-encodedtransmitter as in a PSPON, the present invention uses a multiwavelengthtransmitter in the central office (CO) and sends multiple wavelengths tothe PSPON. There is no change to the remote passive power-splittingnode. Each optical network unit (ONU) at a remote unit receives all thewavelengths. A filter is placed in each ONU so that each remote unitreceives a different wavelength. This effectively acts as a WDM PON at amuch lower cost than a true WDM PON. The ONUs are preferably identical.

The present invention uses flexible TDM bandwidth allocation, as in aconventional PSPON system. Upstream is still shared unless otherwiseprovided. The access provider provides data at different bandwidths, andeach remote unit receives its data from one of these bandwidths. No tworemote units receive the same bandwidth.

FIG. 2 shows an exemplary network system in which an access provider 20has access to all the remote units 40, 42, 44 through a power splitter30. For example, the provider 20 provides signals at multiple bandwidths(or wavelengths) to a power splitter 30. The power splitter 30distributes the signal to every remote unit 40, 42, 44 in the system.Each remote unit 40, 42, 44 having the proper receiving equipmentreceives the appropriate signal.

FIG. 3 shows another exemplary network system in which an accessprovider has access to all the remote units 80, 82, 84 in a system. Theaccess provider transmits using a distributed feedback laser 52 whichtransmits at different wavelengths λ₁, λ₂, λ_(N). The optical signalsare passed through a multiple channel filter device such as a waveguidegrating router or power splitter 60 and passed to the optical fibers 65comprising a distribution system. The signals are received at a powersplitter 70 (e.g., a 1×M power splitter) and provided to the remoteunits 80, 82, 84 which have an appropriate filter to receive the desiredsignals and filter out the unwanted or unauthorized signals. An upstreamshared time division multiplexer 90 is also shown and is used totransmit optically encoded signals upstream to the remote node to form amultiplexed signal for distribution to the CO.

Each remote unit has equal access to the bandwidths using ONU filtering.Optical filtering at the ONU provides flexibility. The PSPON sends allthe wavelengths to all the customers on a PON. The access provider usesdifferent wavelengths to transmit its data, as shown for four exemplarybandwidths in FIG. 4A. The provider transmits data at a first bandwidthof wavelength λ₁, at a second bandwidth of wavelength λ₂, at a thirdbandwidth of wavelength λ₃, and data at a fourth bandwidth of wavelengthλ₄. Each remote unit receives all the bandwidths and wavelengths, thusall the data, but each remote unit contains a filter so that only thedesired wavelength or data is ultimately received by the user. Forexample, as shown in FIG. 4B, only the data having a bandwidth ofwavelength λ₂ is received by the user after filtering. Thus, the datafrom the access provider is transmitted to each remote unit. At theremote unit, a filter is provided so that the user at the remote unitfilters out the unwanted data and only gets the bandwidth it subscribesto. Thus, the access provider has access to the full downstream PONbandwidth (e.g., 622 Mb/s). The customer can change bandwidth receivedfrom the access provider by changing the optical filter provided on anaccess card.

As described above, each ONU receives all the wavelengths sent by theaccess provider, and a filter is used at the remote unit to provide thewavelengths in the bandwidth that the user seeks access to. Preferably,the filter is incorporated into a card that each user receives. Anexemplary card 100 in accordance with the present invention is shown inFIG. 5. The card 100 incorporates an optical filter 110 and a magneticstripe 120, barcode, or other means for user verification andidentification. Accordingly, wavelength-encoding is possible. Thus, acard with an optical filter in it is provided to the remote end user(e.g., a consumer or access provider subscriber) who uses it to selectthe bandwidth wavelengths or channel that the remote unit is to receive.The filter 110 can be a transmissive or reflective holographic filter, atransmissive or reflective interference filter, or any type oftransmissive or reflective thin-film optical filter. Thus, the user canchange (e.g., upgrade) to different wavelengths by having a differentfilter put on the card; e.g., the user can upgrade from a low cost, lowspeed data channel to a more expensive, high speed data channel.

The user selects the bandwidth to receive by inserting the card into anONU at the remote unit. As described, a thin-film narrowband opticalfilter is laminated into the card, and that portion of the card ispositioned between a fiber exit and a receiver, preferably using agraded index (GRIN) lens. The magnetic stripe is read by the ONU andsends back a signal indicating that the authorized user is receiving thecorrect bandwidth signal. Many other conventional security measurescould be incorporated to deter counterfeiting.

The filter passband is preferably made highly temperature-independent,and designed for high transmittance. The power budget for the presentinvention is not much different than a conventional PSPON becausepresently available centralized PON (CPON) routers specify a 10 dBon-channel loss, compared to a 12 dB loss for a 16-port power splitter.This also eliminates the wavelength routers completely from the CPON,along with a temperature tuning problem that causes many problems.

An exemplary ONU 200 in accordance with the present invention is shownin FIGS. 6 and 7. The broad spectrum output from a suitable opticalsource (not shown)--illustratively, a light emitting diode (LED) havingan output centered at a typical telecommunications wavelength such as,for example, 1.55 μm--provides the incoming data signal to an ONU 200through a fiber 205. The ONU 200 has a card reader 210 in which the userinserts the card 100 containing the filter 110. The card reader 210reads the magnetic stripe 120 or barcode for user identification andverification. The incoming optical signal passes through a GRIN lens 207which acts as a receiver, passes through a dichroic mirror 217, andpasses through the optical filter 110 on the card 100. A detector 220,preferably a 1.55 μm light detector, detects the transmitted opticalsignal that passes through the filter 110 and provides it to the nextpart of the system, for example, a conventional person computerminiature card interface adapter 250 (PCMCIA). A source 215, preferablya 1.3 μm light source, is used for upstream transmission from the remoteunit to the CO.

It should be noted that although an output spectrum centered about 1.55μm is shown and described, it is nonetheless contemplated that theoutput spectrum of the optical source might alternatively be centeredabout some other wavelength of interest such, for example, as 1.3 μm,and that reference herein to any particular wavelength band is by way ofillustrative example only.

An exemplary system in accordance with the present invention is nowdescribed. Assume that a PSPON with a 16-way power splitter is running aTDM data format into identical ONUs, and the total capacity, preferably52 or 155 Mb/s, or any of the low data rate systems as required forlocal access, is shared between the 16 users. This existing PSPON isupgraded to a full WDM PON without changing the outside plant based onthe following.

A multiwavelength transmitter is installed in the CO for each PON andemits signals on a predetermined number of different wavelengths (orwavelength channels) at a predetermined data rate, preferably 16different wavelengths at a 52 or 155 Mb/s data rate per channel, forlocal access applications. Each customer on the PON receives a card thatis to be inserted into the ONU. A small thin-film interference filter islaminated into each card and transmits one of the wavelength bands. Wheninserted, the filter is disposed between the fiber and the detector,preferably using one or a pair of GRIN lenses. The filter peaktransmission is made high (preferably about 80%) and the blocking ofadjacent channels is set greater than 20 dB, with a substantially flattop. The passband of the filter is rendered temperature insensitiveusing conventional methods such as those used in current WDM routers.Therefore, the temperature tuning problem of the WDM routers is notpresent in the exemplary embodiment; therefore, large wavelengthspacings are not required for `set-and-forget` operation.

When the card is inserted, information is read from the card to ensurethat the correct customer is receiving the correct data. Conventional"Smart Card" security features can be incorporated to preventcounterfeiting.

Because the filter passes WDM formatted data, time-demultiplexingelectronics is not needed in the ONU. Each ONU receives the full 52 or155 Mb/s, and can be further upgraded to a high data rate system of 622Mb/s, if desired.

With respect to the total loss, a 16-way power split causes a 12 dBloss, and conventional WDM routers cause a 10 dB loss, therefore as longas the filter transmission is nearly 100% at peak, the loss budgets arecomparable. In a PSPON, receivers operate at 155 MHz or greater.

In another exemplary embodiment, instead of putting the filters insidethe ONUs, the filters are placed inside the remote splitting node. Thefilters preferably transmit the 1.3 μm light also. Such two-band filterscould be made with a slightly more complex layer design, however gettingout of the fiber and back into the fiber is much more complex andexpensive than just putting the interference filter in front of thephotodetector in the ONU, especially if the remote splitter has to be arugged unit that hangs on the side of a telephone pole.

In another exemplary embodiment, as shown in FIG. 8, the entire ONU isput into a PCMCIA-type card 300, so that it incorporates all thefeatures of the ONU, including the filter 110, and can be easilyexchanged by the user. The PCMCIA card contains the optical filter 110and the customer registration information. Because this is WDM, no timedemultiplexing electronics is required, thereby simplifying and reducingpower requirements. Also provided, though optional, is a POTS (plain oldtelephone service) input 290. It should also be noted that the upstreamlaser diode would have to be built into this unit, also.

Although illustrated and described herein with reference to certainspecific embodiments, the present invention is nevertheless not intendedto be limited to the details shown. Rather, various modifications may bemade in the details within the scope and range of equivalents of theclaims and without departing from the invention.

What is claimed is:
 1. A method of operating a passive optical networkproviding a predetermined wavelength of data to remote users comprisingthe steps of:a) providing, by an access provider, an opticalmultiwavelength signal having a plurality of signal components each ofpredetermined wavelength; b) transmitting the multiwavelength signal; c)receiving and power-splitting the multiwavelength signal into aplurality of distributed multiwavelength signals each associated with arespective remote user; and d) selectively filtering out, for eachremote user, one of the signal components of the associated distributedmultiwavelength signal to provide the remote user with a selected onesignal component of predetermined wavelength; wherein step (d) furtherincludes providing a card containing an optical filter, inserting thecard into a remote optical network unit associated with the remote user,and positioning the card so that the optical filter is in position tointercept the respective multiwavelength signal and pass the selectedone signal component.
 2. The method of operating a passive opticalnetwork according to claim 1 in which step d) includes controlling theselective filtering to filter out another one of the signal componentsto provide the remote user with a different selected one signalcomponent of predetermined wavelength.
 3. The method of operating apassive optical network according to claim 2 in which step d) furtherincludes providing the card with a program for the controlling of theselective filtering.
 4. The method of operating a passive opticalnetwork according to claim 1 in which step a) includes using adistributed feedback laser for providing the plurality of signalcomponents each of predetermined wavelength.
 5. The method of operatinga passive optical network according to claim 1 wherein the remote userselects a different one signal component of predetermined wavelength byinserting a different card into the remote optical network unit.
 6. Themethod of operating a passive optical network according to claim 1further comprising the step of:e) identifying the remote user asauthorized to receive the selected one signal component of predeterminedwavelength before transmitting the multiwavelength signal.
 7. A passiveoptical network system for providing a predetermined wavelength of datato remote users comprising:a multiple wavelength transmitter fortransmitting a multiwavelength signal having a plurality of signalcomponents each of predetermined wavelength, the multiwavelength signalbeing provided by an access provider; a power-splitting passive opticalnetwork for receiving and power-splitting the multiwavelength signalinto a plurality of distributed multiwavelength signals each associatedwith a respective remote user; and a filter provided on a card forselectively filtering out, for each remote user, one of the signalcomponents of the associated distributed multiwavelength signal toprovide the remote user with a selected one signal component ofpredetermined wavelength; wherein the card is inserted into a respectiveremote optical network unit and positioned to intercept themultiwavelength signal for selectively passing the selected one signalcomponent.
 8. The passive optical network system according to claim 7wherein said multiple wavelength transmitter includes a distributedfeedback laser for providing the plurality of signal components each ofpredetermined wavelength.
 9. The passive optical network systemaccording to claim 7 further comprising a plurality of remote opticalnetwork units each associated with a respective remote user andreceiving a respective distributed multiwavelength signal from thepower-splitting passive optical network.
 10. The passive optical networksystem according to claim 7 wherein the card is programmed forcontrolling of the selective filtering.
 11. The passive optical networksystem according to claim 10 wherein the card identifies the remote useras authorized to receive the selected one signal component.
 12. Thepassive optical network system according to claim 11 wherein the cardincludes means for identifying the remote user.
 13. The passive opticalnetwork system according to claim 12 wherein the means for identifyingis selected from the group consisting of an optical filter, a magneticstrip, and a barcode.
 14. The passive optical network system accordingto claim 7 wherein the remote optical network unit is situated within apersonal computer miniature card interface adapter (PCMCIA).
 15. Thepassive optical network system according to claim 7 wherein the filteris selected from the group consisting of a transmissive filter, areflective filter, a holographic filter, an interference filter, and anoptical filter.